Infection with the small flagellate Giardia lamblia is found around the world. This protozoan inhabits the small intestine of humans and is especially common in children. Other mammals, including beavers and muskrats, also harbor Giardia and are important reservoir hosts. The parasite was first seen by Anton van Leeuwenhoek in 1681 and described scientifically in 1859.

Adult parasites, the trophozoites, attach to the intestinal wall with sucking disks. As trophozoites detach and pass down the intestinal tract, they transform themselves into cysts that are able to resist many environmental pressures, including water filtration and chlorination. Humans almost always acquire infection by swallowing fecally contaminated food or water. In developed countries, many cases of giardiasis have been traced to campers who have drunk from what appeared to be pure wilderness streams, but that had been contaminated by animals. Because the cysts are surprisingly resistant to normal water purification methods, public water supplies can become infected by faulty sewer lines, as happened in two fashionable Colorado ski resorts in 1964 and 1978. Giardiasis is a frequent cause of “traveler’s diarrhea,” and tourist groups in Leningrad have suffered well-publicized outbreaks. In 1983, 22 of New York City’s 55 police and fire department scuba divers had Giardia, presumably from the heavily polluted waters of the harbor. Four percent of 1 million stool samples submitted to state laboratories in the United States from 1977 to 1981 were positive for Giardia. Prevalence rates in developing countries range from 8 to 20 percent and higher.